1. Technical Field
The present invention relates to a capacitance type sensor device in which a physical amount is detected in accordance with a change of an electrostatic capacitance.
2. Prior Art
As a conventional capacitance type sensor device, a failure diagnosis device having an electrostatic capacitance type sensor device is disclosed, for example in Japanese patent laid-open No. 223,844/1993, in which a capacitance type sensor device linearly detects a sensor capacity.
The above stated conventional capacitance type sensor device is shown in FIG. 2. In FIG. 2, a detector comprises switches 10 and 12, a sensing part 11, a condenser 13, an operational amplifier 14, and a sample hold circuit 15.
An electrostatic capacitance of the sensing part 11 is charged and discharged by the switch 10, and this current flowing charge and discharge current is charged by an integrator having a reset function, this integrator comprises the switch 12, the condenser 13, and the operational amplifier
Accordingly, as shown in FIG. 3, a pulse-wise wave-form having a wave height value in response to a capacitance value of the sensing part 11 is obtained as an output of the operational amplifier 14. By sampling the wave height value of this pulse-wise wave-form through the sample hold circuit 15, a change of the electrostatic capacitance is detected.
However, in the conventional capacitance detector, the output of the operational amplifier 14 changes with a pulse-wise wave-form as shown in FIG. 3. As a result, a very fast response performance and a large output current is required in the operational amplifier 14.
Further, since the output of the operational amplifier 14 is the pulse-wise wave-form, the sample hold circuit 15 is provided at a latter stage, thereby it is necessary to convert to a continuous signal. Accordingly, in the above stated conventional technique capacitance detector, a large circuit scale is needed.
Further, in a case that the capacitance detector of the conventional technique is assembled, MOS capacitance is used as the condenser 13 for determining a sensibility of the capacitance detector. However, by a voltage dependency of MOS capacitance, a large non-linear characteristic causes in a relationship between a capacitance value of the sensing part 11 and the output voltage of the capacitance detector.
Further, so as to raise the sensibility of the capacitance detector according to the conventional technique, it requires to make small the capacitance value of the condenser 13.
However, a very large offset capacitance (an electrostatic capacitance part not be changed according to the detected physical amount) exists at the sensing part 11, when the capacitance value of the condenser 13 is made small, a stability performance or a response performance of the operational amplifier 14 becomes low, consequently there is a limitation for rasing the sensibility.